It has been recognized, especially in the actuation or control of aircraft primary flight control surfaces, that failure modes which result in the surface being jammed can cause a flight safety problem. System failures, such as a loss of voltage or extremely high voltage, could cause a command signal to actuate the control surface to an extreme position which also would result in a critical flight safety situation. Actuators move or control such control surfaces as rudders, horizontal stabilizers, ailerons, and the like.
Traditionally, flight control systems for commercial and military aircraft primarily have employed hydromechanical actuation systems due to the reliable, non-jamming high response characteristics of hydraulic cylinders. Advancements in electronics and the development of rare earth permanent magnet brushless DC motors, with their characteristic high torque to inertia ratio, have led the trend toward digital fly-by-wire flight control systems controlled by computers versus the traditional hydraulic modes. The advantages of such advanced fly-by-wire flight control systems include reduced weight and maintenance due to the elimination of fluid plumbing throughout the aircraft, and active flight control for stability augmentation and ride quality enhancement.
However, the advanced electromechanical actuation systems use geared rotary or linear ballscrew or Acme threaded actuators which often are considered unacceptable for many applications due to failure conditions where the system can jam in a fixed position. This results in a jammed control surface, and there also is concern about ballscrew wear, response speeds and actuator stiffness. Of course, jamming is of critical importance because of the safety problem involving loss of control of the aircraft.
This invention is directed to solving the above problems by combining the advantages of electromechanical actuation system fly-by-wire technology with a reliable, non-jamming, high response feature incorporated in the system. The above-referenced copending application discloses a novel non-jamming ballscrew actuator which includes a closed hydraulic circuit providing the driving link for the actuator, the circuit being selectively openable in the event of jamming of the ballscrew to allow reaction movement of the actuator in response to load forces on the control surface. The present invention is directed to a new and improved non-jamming mechanical ballscrew actuator system which provides for driving the actuator notwithstanding jamming of the ballscrew assembly but which also allows for reaction movement of the actuator means in respone to load forces in the event of failure of the system upstream of the ballscrew assembly.